Several trends exist presently in the semiconductor and electronics industry. Devices are continually getting smaller, faster and requiring less power. A reason for these trends is that more personal devices are being fabricated that are relatively small and portable, thereby relying on a battery as their primary supply source. For example, cellular phones, personal computing devices, and personal sound systems are devices in great demand in the consumer market. In addition to being smaller and more portable, personal devices are requiring more computational power and speed. In light of all these trends, there is an ever increasing demand in the industry for smaller and faster transistors used to provide the core functionality of the integrated circuits used in these devices.
Accordingly, in the semiconductor industry there is a continuing trend toward manufacturing integrated circuits (ICs) with higher transistor densities. To achieve high densities, there has been and continues to be efforts toward scaling down dimensions (e.g., at submicron levels) on semiconductor wafers, which are generally produced from bulk silicon. In order to accomplish such high densities, smaller feature sizes, smaller separations between features and more precise feature shapes are required in integrated circuits (ICs) fabricated on small rectangular portions of the wafer, commonly known as dies. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, as well as the surface geometry of various other features (e.g., corners and edges). The scaling-down of integrated circuit dimensions can facilitate faster circuit performance and/or switching speeds, and can lead to higher effective yield in IC fabrication by providing more circuits on a die and/or more die per semiconductor wafer.
The process of manufacturing integrated circuits typically consists of more than a hundred steps, during which hundreds of copies of an integrated circuit can be formed on a single wafer. This process can create electrically active regions in and on the semiconductor wafer surface. In forming metal oxide semiconductor (MOS) transistors, for example, a gate structure is created, which can be energized to establish an electric field within an underlying semiconductor channel, by which current is enabled to flow between a source region and a drain region within the transistor. The source and drain regions facilitate this conductance by virtue of carefully tailored doping to form positively doped (p) or negatively doped (n) regions around the channel.
Preferably, the gate structure is topped off with a conductive contact having a low resistance. Such a contact can comprise a silicide, which is produced by an interaction between metal and silicon or polysilicon to produce a metal-silicon alloy. The process of forming a silicide is known as silicidation, and generally includes some type of heat treatment (e.g., annealing, sintering) to cause the metal and silicon to react with one another. Silicides generally have a low resistivity and thus perform well as gate contacts in transistors. A salicide is a self-aligned silicide formed atop a silicon gate. The silicide is said to be self-aligned, or a salicide, because it only reacts with the underlying silicon gate structure and thus does not extend off onto other structures, such as insulative sidewall spacers.
To activate a MOS transistor, a voltage is applied to the gate structure via the conductive contact. Such a voltage is referred to as a threshold voltage (VT). The value of the threshold voltage is an important parameter in transistor circuit performance. A lower VT means less power has to be supplied to a transistor circuit for activation, allowing the circuit to react quicker and operate faster. A primary parameter that determines the threshold voltage is the work function of the circuit. The work function can be thought of as a kind of electrical compatibility: the lower the work function, the lower the threshold voltage, the lower the power required to run the circuit, etc. The conductivity and resistivity of the gate contact affects the work function. In particular, the more conductive and less resistive the gate contact, the lower the work function, and the faster and more responsive the circuit. Additionally, multiple devices are commonly formed within integrated circuits, and such devices may require different silicides to provide different performance characteristics.
Accordingly, improved techniques for efficiently fabricating densely packed semiconductor devices with salicide gate contacts that yield desired work functions and operating characteristics would be desirable.